1. Field of the Invention
The invention relates to buildings and, more particularly, to an improved wallboard panel for use in framed residential and commercial buildings.
2. Description of Prior Art
Since the time of the Industrial Revolution, framed building structures have a played a vital role in fulfilling the housing needs of an expanding U.S. population. It was in the 1830's that the first “Balloon Framed” structure was developed. Coupled with the means to mass produce nails, this new form of framing enabled builders to construct a home far more rapidly and for less money than the previous method of building which was referred to as “Post & Beam” or “Post-Frame” construction. A short time later, around the 1850's, “Balloon Framing” evolved into “Platform Framing.” And to this day, well over a century later, the platform framing method is still employed as the best means for constructing homes in terms of both time and money savings.
Bracing these new light framed structures for stability was recognized as an essential element necessary for the safety of the occupants. Transient horizontal forces acting on these structures like those generated by wind and earthquakes were dealt with in much the same way until the 1960's. That is, between the 1830's and late 1950's, both the balloon framed structures as well as the platform framed structures (hereinafter referred to as “light framed structures”) were braced with 1×6 sheathing boards fastened with two nails to each stud. Later, around the 1960's, the 1×6 sheathing boards were replaced with sheets of plywood. Basically, builders recognized that a single sheet of plywood could replace 16-1×6's and required half the amount of nails to provide at least the same level of strength and rigidity. This has been verified by many tests proving sheathed vertical diaphragms, also known as shear walls in light framed construction, can absorb a great deal of energy before failure making them very ductile and reliable. Plywood and its derivatives, such as oriented strand board, are still employed today as the best means for bracing homes in terms of both time and money savings but are nonetheless a combustible element not well suited as a substrate for many wall finishes.
Interior finishes for these new light framed structures consisted of plaster on lath until the first drywall panels were developed in the late 1910's by the United States Gypsum Company. However, it wasn't until the end of the Second World War, around the 1940's, that gypsum wallboard held an established place in the market as the best means for finishing walls. Today these panels have been refined and reformulated to be lighter and more fire resistant than their predecessors. In fact, they are recognized by modem building codes as the preferred cladding material for making light framed structures resistant to fire damage. Gypsum wallboard's effectiveness as a fire resistive barrier is due largely to its water content. When exposed to fire, these panels slowly release their water content in the form of steam, a process referred to as calcination, thereby retarding heat transmission to the underlying framing. After calcination is complete these panels continue to act as a barrier protecting the underlying framing from direct exposure to flames. These gypsum-based panels, however, lack the necessary in-plane shear strength, rigidity and ductility to adequately brace light frames structures against wind and earthquake forces. In some areas of the United States, where earthquakes are severe, their use as a shear wall sheathing element is strictly prohibited by the building code. This is due largely to the brittle, low strength nature of these panels. As a result, it has become a convention in modem construction to brace light frames structures by attaching a layer of plywood sheathing to the wall framework first. Then a layer of gypsum wallboard panels is attached over the plywood to finish the walls and protect the underlying framing and plywood from fire damage.
For the most part, thermal insulation for the exterior walls of these light framed structures consisted of a layer of building paper between the 1×6 sheathing and exterior siding until the 1930's. It was then that concerns over the cost of energy bills emerged and various insulation products such as mineral wool and eelgrass in paper quilts were developed to reduce coal bills. Today conventional insulation in light framed structures mostly involves filling the cavity between the wall studs with mass consisting of fiberglass, cellulose, or rock wool which slows heat flow resulting from conduction and convection. Disadvantageously, these mass insulation systems have little effect on heat flow resulting from radiation, more specifically, infrared radiation. It is well known that conduction, convection, and (infrared) radiation comprise the three modes in which heat transfer takes place. It is further well known that the addition of a reflective insulation system could prevent up to 65% of heat loss through walls during winter and 85% of heat gain during summer but are seldom installed in modem construction because of the added time and cost. In general, these reflective insulation systems consist of at least one layer of a low-emissivity material, such as aluminum foil, adjacent to an air space. It is also well known that these aluminum foils are typically between 0.00025 inches and 0.0004 inches thick. They're kept especially thin to save money but also because they're not intended to be used as a structural element. With rising concerns over high energy costs, as well as, the consumption of fossil fuels and the “greenhouse” gases they produce, insulating homes effectively to reduce the energy needed for their heating and cooling systems is both economically and ecologically essential.
Today, where affordable housing is in great demand, novel approaches are still needed that continue to simplify construction thereby saving time and money. This can be a daunting challenge given the increased performance expectations imposed on buildings by modem building codes. In addition, solutions are needed that also address energy efficiency over the life of the structure as well as the immediate cost savings sought during construction. After all, statistics show that between 50% and 70% of the energy used in the average home in the United States and Canada is for heating and cooling. Excluding the present invention, no wallboard panel exists today that is readily handled and quickly attached to the framework of light framed structures for the purpose of bracing, finishing, fireproofing, and thermally insulating wall structures in a single sheathing procedure. Such a composite panel would simplify construction by eliminating the need for multiple sheathing procedures thereby saving time and money. And such multiple sheathing procedures are currently needed since gypsum wallboard lacks the strength and rigidity, and in some cases prohibited, to be used as a shear wall sheathing element for resisting in-plane shear loads from wind and earthquake forces. Additionally, the same composite panel would address the need for coupling a reflective insulation system with a conventional mass insulation system thereby addressing all three modes of heat transfer. The end result being added long-term cost savings from lower heating and cooling bills as well as conservation of fossil fuels and a reduction of the “greenhouse” gases produced.
The prior art which most closely resembles the present invention is U.S. Pat. No. 5,768,841 to Swartz et al. which discloses a steel sheet attached to a gypsum wallboard panel where the metal sheet resists in-plane shear loads imposed on the framing structure of the building due to exterior environmental conditions. Disadvantageously, steel is heavy and its density can add significant weight to a gypsum wallboard panel (e.g. the density of steel is three times greater than aluminum). A typical sheet of ⅝″ gypsum wallboard covering a wall area of 32 square feet weighs about 90 pounds each (conventional panels are typically manufactured with rectangular extents of 4 feet by 8 feet). Adding the weight of a steel sheet, as disclosed in the aforementioned patent, would increase the weight of the composite panel by at least 20 pounds but by as much as 80 pounds. That means the combined weight of the steel sheet and gypsum wallboard panel would weigh between 110 and 170 pounds. Added worker fatigue and a change in the way panels are normally handled would probably occur as a result of the weight of these unwieldy steel sheet composite panels. The end result being a slower installation process adding labor time and cost to construction.
Further, it is believed that a sheet of aluminum, for example, whose strength is less than the steel sheet disclosed by Swartz et al. is capable of resisting shear loads imposed on the shear wall due environmental conditions such as wind and earthquakes. Further yet, it is believed that a sheet of aluminum whose strength is less than the steel sheet disclosed by Swartz et al. can yield in-plane shear load capacities and rigidities relatively close to the capacities and rigidities expected from plywood or oriented strand board if its thickness is within the range of 0.006 and 0.03 inches and it possesses a minimum tensile strength of 20,000 pounds per square inch. It's even doubtful that Swartz et al.'s invention could achieve in-plane shear load capacities significantly greater than those expected from plywood sheathed shear walls. The surprising reason for this has to due with the fasteners used to attach the plywood panels to the framework, and not the plywood panels themselves. Shear walls, in modem light framed structures, are a complex assembly comprised of wall framework such as wall studs, sheathing panels such as plywood, and fasteners such as nails or screws. Tests on these shear wall assemblies have shown that the fasteners tend to pry loose from the underlying framework before the plywood panels rupture. Therefore, it is not advantageous to use a metal sheet whose in-plane strength and rigidity are greater than that of a sheet of plywood.
Another consideration regarding the use of steel sheets in a composite panel application would be the corrosive nature of steel. Left unprotected, steel will rust which lowers its' structural integrity. To protect steel against the harmful effects of rusting it is usually dipped in a zinc-based coating to shield it from the oxygen and moisture in our atmosphere. Steel can also be made corrosion resistant by alloying it with other metals like chromium and nickel. Although either method can adequately protect steel from corrosion, they disadvantageously add significant cost to the price of using steel.
Yet another disadvantage regarding the use of steel sheets in a composite panel application would be the high-emissivity properties of steel. Steel, like most building materials, tends to readily absorb and emit radiant heat. Radiant heat, commonly referred to as infrared radiation, can be defined as a band of electromagnetic waves in the wavelength range of 4 to 40 microns. These electromagnetic waves are constantly passing through the air, traveling in a straight line between the surfaces of wanner objects they're emitted from while heating the surfaces of the cooler objects they strike. All objects both absorb and emit infrared radiation, however, some absorb and emit substantially less due to their unique emissivity properties and surface characteristics. For example, materials said to have an emissivity factor of no greater than 20% would be said to have low-emissivity properties. To elaborate, these low-emissivity materials would neither absorb nor emit more than 20% of their heat through radiation. Reflective insulation systems, as mentioned previously, require at least one layer of a low-emissivity material, such as aluminum foil, to substantially impede radiant heat transfer. Remembering that light framed structures tend to lack a means of addressing heat transfer due to radiation, steel would not offer a solution to this insulation shortcoming. Aluminum foil, on the other hand, is well known to offer a solution to this insulation shortcoming but lacks the necessary strength and rigidity to reinforce a gypsum wallboard panel for resisting in-plane wind and earthquake loads adequately because it's too thin. According to the “Aluminum Standards and Data” handbook published by the Aluminum Association. “Foil-” is defined as “a rolled product that is rectangular in cross section of thickness less than 0.006 inch.” However, it's well known that aluminum foils used for this application are typically between 0.00025 inches and 0.0004 inches thick which is more then ten times thinner than the aluminum sheet needed to provide the necessary in-plane shear strength and rigidity for resisting wind and earthquake loads adequately.